The present invention relates to a pressure-type flow controller for a variety of fluids, like gases used in the manufacturing of semiconductors, chemical products, chemicals and precision machine parts. More specifically, this invention relates to a method of detecting abnormalities in flow rate when an orifice is clogged in the pressure-type flow controller.
Hitherto, the flow rate has been controlled mostly by a mass flow controller in fluid supply apparatuses in semiconductor manufacturing and chemicals manufacturing facilities, especially where a high precision flow rate control is required.
However, the mass flow controller has the following shortcomings:
(1) relatively slow response in case the sensor is of a thermal type, (2) poor control precision in a low flow rate range and product-to-product lack of uniformity, (3) trouble-proneness in operation, that is, low reliability, (4) high costs of the product and replacement parts which boost running costs.
After intensive research, the inventors have developed a pressure-type flow controller using an orifice as disclosed in Japanese Laid-open Patent Application No. 8-338546.
The principle of this pressure-type flow controller is the following. If the ratio P2/P1 of the gas pressure on the upstream side of the orifice P2 to that on the downstream side of the orifice P1 is below the critical pressure of the gas (in the case of air, nitrogen etc., about 0.5), the flow velocity of the gas passing through the orifice reaches a sonic velocity so that changes in the pressure on the downstream side of the orifice will not be felt on the upstream side any more, which permits a stable mass flow rate corresponding to the state on the upstream side of the orifice.
If the orifice diameter is fixed or constant and the pressure P1 on the upstream side is set at twice or more as high as the pressure P2 on the downstream side, the gas flow rate QC on the down stream side of the orifice depends on only the pressure P1 on the upstream side. That is, a linear relation given by equation QC=KP1 (K is a constant) holds with a high degree of precision. Moreover, if orifices have the same diameter, then they are also identical in constant K.
This pressure-type flow controller has an advantage that the flow rate on the downstream side can be controlled with high precision by detecting the pressure P1 on the upstream side only.
However, a problem is that the small size bore of the orifice can clog. The orifice bore is in the order of microns, and it can happen that the orifice bore gets clogged by dust or the like, which renders the orifice uncontrollable.
The piping in which the flow rate is controlled has to be highly clean inside, but sawdust and other foreign matter can creep in during the piping work. If the orifice is clogged, the flow rate can not be controlled, which puts the whole plant in an unstable state and can produce a large number of defective products.
With some gases, the chemical reaction becomes uncontrollable and could cause an explosion. To prevent the clogging, it might be suggested that a gasket filter should be placed within the piping. But that could have an adverse effect on the conductance of the piping.
As a solution to the problem, the inventors disclosed a method of detecting the clogging in unexamined Japanese patent application No. 10-236653. In this method, an alarm is activated when the clogging of the orifice reaches a specific level.
The aforesaid invention comprises, as shown in FIG. 8, a first step of holding a set flow rate QS at a high set flow rate QSH (usually 100%), a second step of obtaining pressure attenuation data P(t) by switching from the high set flow rate QSH to a low set flow rate QSL (usually, 0%) to measure the pressure P1 on the upstream side, a third step of checking the aforesaid attenuation data P(t) against reference pressure attenuation data Y(t) measured under the same conditions with the orifice not clogged, and a fourth step of setting off an alarm when the pressure attenuation data P(t) differ from the reference pressure attenuation data Y(t) by a specific quantity. That is, in case P(t) is out of Y(t) within a certain range, an alarm for the clogging will be activated.
The aforesaid method offers an advantage that the alarm for the clogging can be automated, but has the following shortcomings.
First, the flow rate abnormality detection mode (clogging detection mode) is started after the flow control mode is shut off, and therefore the flow rate can not be controlled during the flow rate abnormality detection mode.
Another problem is that since the low set flow rate QSL is set at 0%, for example, time is needed for exhausting. That is, it takes long to make a judgment. Therefore, the detection of a flow rate abnormality is generally carried out in a final stage after the flow rate control is completed. As a result, it is impossible to know, in the actual controlling of the flow rate, whether the orifice has been clogged.
The present invention sets out to solve those problems. In accordance with the present invention, in a pressure-type flow controller having a control valve, an orifice, a pressure detector detecting the pressure on an upstream side between the control valve and the orifice, and a flow rate setting circuit, wherein the upstream pressure P1 is maintained about two or more times higher than the downstream pressure P2, the downstream flow rate QC is calculated by the equation QC=KP1 (K: constant), and wherein the control valve is controlled on the basis of the difference signal QY between the calculated flow rate QC and the set flow rate QS, there is provided a method according to the present invention of detecting the clogging of the orifice in the pressure-type flow controller. The method of the invention comprises, with a testing circuit provided separately and with the flow control of the control valve placed in a shut off state, outputting a testing signal xcex94QS having a testing amplitude VO from a testing circuit to the control valve, measuring a pressure amplitude V of a variable pressure xcex94P1 of the pressure P1 on the upstream side that arises in response to opening and closing adjustment of the control valve and setting off an alarm for the clogging of the orifice when the pressure amplitude V is smaller than a limit amplitude Vt.
The present invention provides a second method of detecting flow rate abnormality which comprises, without a testing circuit separately provided, outputting a testing signal xcex94QS having a testing amplitude VO from a flow rate setting circuit 32, measuring the pressure amplitude V of variable pressure xcex94P1 of the pressure P1 on the upstream side that arises in response to the outputting, and setting off an alarm for the clogging of the orifice when the pressure amplitude V is smaller than the limit amplitude Vt.
The present invention also provides a method of detecting a flow rate abnormality, comprising: outputting a testing signal xcex94QS to a control valve CV with the testing signal xcex94QS superimposed on a signal representing a steady-state set flow rate QSO.
The present invention furthermore provides a method of detecting a flow rate abnormality wherein the testing signal xcex94QS to be outputted to a control valve CV is a sine wave signal.
The present invention also provides a method of detecting a flow rate abnormality wherein a testing signal xcex94QS to be outputted to a control valve CV is a pulse signal.